This invention relates in general to insulation products made from fibrous minerals like glass and, in particular, to quality control methods for determining the cure status, i.e. whether the product is undercured, overcured or properly cured within specifications and process control limits.
Fibrous glass insulation products generally comprise randomly-oriented glass fibers bonded together by a cured thermosetting polymeric material. Molten streams of glass are drawn into fibers of random lengths and blown into a forming chamber or hood where they are randomly deposited as a pack onto a porous, moving conveyor or chain. The fibers, while in transit in the forming chamber and while still hot from the drawing operation, are sprayed with an aqueous dispersion or solution of binder. The residual heat from the glass fibers and combustion gases, along with air flow during the forming operation, are sufficient to vaporize and remove much of the sprayed water, thereby concentrating the binder dispersion and depositing binder on the fibers as a viscous liquid with high solids content. Ventilating blowers create negative pressure below the conveyor and draw air, as well as any particulate matter not bound in the pack, through the conveyor and eventually exhaust it to the atmosphere. The uncured fibrous pack is transferred to a drying and curing oven where a gas, heated air for example, is blown through the pack to dry the pack and cure the binder to rigidly bond the glass fibers together in a random, three-dimensional structure, usually referred to as a “blanket.” Sufficient binder is applied and cured so that the fibrous pack can be compressed for packaging, storage and shipping, yet regains its thickness—a process known as “loft recovery”—when compression is removed.
While manufacturers strive for rigid process controls, the degree of binder cure throughout the pack may not always be uniform for a variety of reasons. Irregularities in the moisture of the uncured pack, non-uniform cross-machine weight distribution of glass, irregularities in the flow or convection of drying gasses in the curing oven, uneven thermal conductance from adjacent equipment like the conveyor, and non-uniform applications of binder, among other reasons, may all contribute to areas of over- or under-cured binder. Thus it is desirable to test for these areas in final product to assure quality.
U.S. Pat. No. 3,539,316 to Trethewey and U.S. Pat. No. 4,203,155 to Garst both describe curing ovens in which a thermocouple is installed inside the curing oven and is used to provide feedback to the heater control to make adjustments if the sensed temperature is not at a predetermined setpoint. While useful, this approach has drawbacks in that the thermocouple senses the generalized oven air temperature and gives no information about the pack temperature where the binder is located, and therefore no information about cure status.
U.S. Pat. No. 7,781,512 to Charbonneau, et al, describes two mechanisms for monitoring the cure status of formaldehyde-free glass fiber products. In the first embodiment, one or more spectrographic sensors, such as an infrared sensor, detect the radiant energy from the pack upon exit from the oven. In a second embodiment, thermocouples are placed directly into the pack prior to entering the oven, and the signals are led by wires to an external device or to a transportable storage device such as a M.O.L.E® recorder (although the term “oven mole” is often used generically). Upon exit, data collected in the storage device is uploaded and in all cases, the measured temperatures are compared to standard values to determine cure.
These methods also have drawbacks. While a “mole” provides a good estimate of the actual pack temperature, it has several disadvantages. First, it measures the temperature at only one location of the pack, testing only a sampling of the product. Second, it must be inserted prior to the oven and removed after the oven, and this involves a labor intensive manual process. Third, it does not provide real-time data; the storage device is removed and evaluated, but this is long after the pack has emerged so the data cannot effectively be used as a means to adjust any process parameters. Finally, it provides data only for as long as the pack is in the oven. In other words, the data it provides is not continuous. On the other hand, infrared measurements may be continuous, but are less useful as process controls when measures after exit from the oven.
The present invention seeks to overcome these disadvantages.